1. Field of the Invention
The invention involves a process for making semiconductor devices with electroless gold layers.
2. Description of the Prior Art
Gold is extensively used in semiconductor devices both as an electrical connection to the semiconductor (for example, as an electrode) and as a heat conducting path for attachment to a heat sink. Advantages in the use of gold are high electrical and heat conductivity, good solderability, freedom from surface corrosion and long-term stability. In the fabrication of semiconductor devices, it is highly desirable to have good adherence of the gold film to the semiconductor surface and that the good adherence remain stable with time. It is also desirable that under manufacturing conditions, the gold film be applied rapidly, simply and inexpensively. Conventionally, gold films are made by evaporation onto the semiconductor surface. Often, chromium or titanium are used as undercoats to insure good adhesion of the gold film to the semiconductor surface. Although these methods often yield perfectly satisfactory results, it is highly desirable to have simpler, less expensive and more reproducible procedures for making adherent gold films. For example, evaporation procedures require vacuum conditions which greatly complicate commercial manufacturing procedures. The need for simpler procedures is particularly evident for electronic and laser applications where masking and etching procedures are used and where small dimensions and close tolerances are characteristic of the device. Formulations for electroless gold baths are known as well as procedures for applying electroless gold to catalytic surfaces (see U.S. Pat. No. 3,700,469).